This invention relates to a high-frequency switch for switching the signal route in a high-frequency circuit such as a portable telephone.
FIG. 7 shows a prior art high-frequency switch 35 having ports P1 and P2 connected by a series connection of a capacitor 31a, a strip line 31a and another capacitor 31b, a junction point between the capacitor 31a and the strip line 31a being connected to the anode of a diode 33a, the cathode of the diode 33a being grounded through a grounding capacitor 31c and also connected to a control terminal Vc1 through a resistor 34a. A junction point between the capacitor 31b and the strip line 31a is not only grounded through another strip line 32b but also connected to a third port P3 through a series connection of another strip line 32c and another capacitor 31d, a junction point between the capacitor 31d and the strip line 32c being connected to the anode of another diode 33b, and the cathode of this diode 33b being not only grounded through another grounding capacitor 31e but also connected through another resistor 34b to another control terminal Vc2. The strip lines 31a and 32c are each a 90-degree phase-shifter with a length equal to or less than one quarter of the signal wavelength at a desired frequency. The strip line 32b is either a high-impedance line or a 90-degree phase shifter.
With the high-frequency switch 35 thus structured, if a positive control voltage is applied to the first control terminal Vc1 and a negative control voltage to the second control terminal Vc2, a reverse bias voltage is applied to the first diode 33a to put it in the OFF condition and a forward bias voltage to the second diode 33b to put it in the ON condition. Thus, the impedance of the strip line 32c, which is grounded through the second diode 33b, becomes infinitely large as seen from the second port P2, allowing signals to travel between the ports P1 and P2, but not between the ports P2 and P3. Similarly, if a negative control voltage is applied to the first control terminal Vc1 and a positive control voltage to the second control terminal Vc2, a forward bias voltage is applied to the first diode 33a to put it in the ON condition and a reverse bias voltage to the second diode 33b to put it in the OFF condition. Thus, the impedance of the strip line 31a, which is grounded through the first diode 33a, becomes infinitely large as seen from the second port P2, allowing signals to travel between the ports P2 and P3, but not between the ports P1 and P2. The strip line 32b serves as a DC route for allowing a direct current to travel through the diode 33a or 33b by the control voltage applied to the control terminal Vc1 or Vc2. In high-frequency situations, the impedance of the strip line 32b becomes large as seen from the junction point between the strip lines 31a and 32c so as to reduce the insertion loss and the reflection loss. In summary, the circuit as shown in FIG. 7 serves as a switch, connecting port P2 selectively to port P1 or P2 by the voltages applied to the control terminals Vc1 and Vc2.
With the prior art high-frequency switch 35, however, the isolation between the ports P1 and P2 is as low as 35 dB. If it is used for a synthesizer of a portable telephone, for example, it is not possible to obtain sufficient communication capability. This makes it necessary to provide another circuit for obtaining sufficient isolation or to use a multi-stage switch, but the circuit becomes complicated and the production cost is adversely affected. Moreover, since two voltage sources are required (one positive and another negative) to be applied to the control terminals Vc1 and Vc2, an extra restriction is imposed in the design of the source circuit, and the production cost of portable telephones becomes higher.